PbABCG1 and PbABCG2 transporters are required for the emission of floral monoterpenes in Phalaenopsis bellina.

Terrestrial plants emit volatiles into the atmosphere to attract pollinators and the enemies of herbivores for defense. Phalaenopsis bellina is a scented orchid species whose main scent components are monoterpenes, including linalool, geraniol, and their derivatives. Here, we investigated whether ABC transporters are involved in its floral scent emission. We carried out whole-genome identification of ABC transporter-related genes using four floral transcriptomics libraries of P. bellina. We identified 86 ABC subfamily G genes related to terpenoid transport. After comparing the gene expression patterns of P. bellina with that of P. aphrodite subsp. formosana, a scentless species, followed by gene-to-gene correlation analysis, PbABCG1 and PbABCG2 were selected. The temporal expression of both PbABCG1 and PbABCG2 was highly concomitant to that of the key enzyme PbGDPS and the major transcription factor PbbHLH4 in monoterpene biosynthesis, with optimal expression on day 5 post-anthesis. Spatial gene expression analysis showed that PbABCG1 was highly expressed in sepals, while PbABCG2 was expressed in the lip. Subcellular localization with a GFP-fusion protein revealed that both PbABCG1 and PbABCG2 are cytoplasmic membrane proteins. Co-downregulation of PbABCG1 and PbABCG2 using both double-strand RNA interference and tobacco rattle virus-based gene silencing led to a significant decrease in monoterpene emission accompanied by an increase in the internal monoterpene pools. Furthermore, ectopic expression of PbABCG1 and PbABCG2 in an ABC16 - mutant yeast strain rescued its tolerance to geraniol. Altogether, our results indicated that PbABCG1 and PbABCG2 play substantial roles in monoterpene transport/emission in P. bellina floral scent.